interpolation calculator excel

What this interpolation calculator for Excel does

This page gives you a practical way to calculate linear interpolation and immediately move that logic into Microsoft Excel. If you have two known points on a straight-line trend and need a value in between, interpolation is usually the fastest way to estimate the missing result.

Common use cases include engineering tables, temperature conversions, calibration curves, finance assumptions, and estimating values from sparse datasets. Instead of manually retyping equations each time, this calculator gives you:

• A numeric interpolation result
• A step-by-step equation breakdown
• An Excel-ready formula using your cell references
• A FORECAST.LINEAR alternative for modern spreadsheets

Linear interpolation formula

The standard formula is:

Where (x₁, y₁) and (x₂, y₂) are your known points, and x is the target input. The output is y, the interpolated value at that target x.

Important: if x₁ = x₂, the formula breaks because you divide by zero. In plain terms, both points would have the same x-location, so a unique slope cannot be determined.

How to interpolate in Excel

Method 1: Direct formula (most transparent)

If your cells are:

• B2 = x₁
• C2 = y₁
• B3 = x₂
• C3 = y₂
• E2 = target x

Use this formula:

Method 2: FORECAST.LINEAR

You can also write:

This is concise and readable, especially if you later expand from two points to larger arrays for trend-based estimates.

Example calculation

Suppose your known points are (10, 100) and (30, 140), and you want x = 18.

• Slope = (140 - 100) / (30 - 10) = 40 / 20 = 2
• Offset from x₁ = 18 - 10 = 8
• Change in y = 8 × 2 = 16
• Interpolated y = 100 + 16 = 116

You can press Load Example in the calculator above to test this exact case instantly.

Interpolation vs. extrapolation in Excel

If your target x lies between x₁ and x₂, you are interpolating. If it is outside that range, you are extrapolating. Extrapolation can still be computed with the same formula, but it is generally less reliable because it assumes the same trend continues beyond known data.

This calculator labels your result clearly so you can spot when you are outside the known interval.

Best practices for spreadsheet accuracy

• Keep x-values sorted in ascending order where possible.
• Use consistent units (for example, all temperatures in °C or all distances in meters).
• Avoid rounding too early; round only final presentation values.
• Add data validation rules in Excel to prevent accidental text inputs.
• Use named ranges for readability in larger models.

Quick FAQ

Can I use this for non-linear data?

You can, but linear interpolation assumes a straight line between points. For curved relationships, use spline fits, polynomial regression, or domain-specific models.

Is FORECAST.LINEAR the same as interpolation?

For two points, it produces the same linear estimate as direct interpolation. With more points, it estimates using linear regression across all known values.

Does this work in older Excel versions?

The direct formula works almost everywhere. FORECAST.LINEAR is available in newer versions; legacy versions may use FORECAST with similar behavior.

Bottom line

If you need an interpolation calculator in Excel format, this page gives you the result and the exact spreadsheet formula you can copy. Use the calculator for quick estimates, then paste the generated formula into your workbook for repeatable, auditable analysis.